Animation stops working when setting master pose component

I have a character mesh with an armature that I’ve created a blueprint for. I would like to attach a secondary skeletal mesh component to this character. I’ve done this many times before with clothing for example, but this time the component mesh I want to attach has exactly the same armature as the character, with some additional bones.

Currently when I press play the character plays the idle animation. The issue is that when I try to set the master pose component, animation stops working and my character defaults to the T-pose. Could this be because the skeletal mesh component I’m attaching has additional bones? The armature is identical otherwise.

In the event that set master pose component can only be used with two meshes with identical skeletons (no extra bones), how could I attach the component mesh? Would this be something FSkeletalMeshMerge could accomplish instead?